1. Field of the Invention
This invention relates to an optical recording medium, a device for recording information on the recording medium and a device for reproducing information from the recording medium. More particularly, this invention relates to an optical recording medium which records information thereon with spatial frequency higher than a normal spatial frequency which is specified by wavelength of a read-out light and numerical aperture of an objective lens, and a device for recording and reproducing information on and from the optical recording medium.
2. Description of Prior Art
As shown in FIGS. 1A and 1B, in order to reproduce information from an optical disc such as a CD (Compact Disc), an LD (Laser Disc) or the like, a read-out laser light is irradiated on pits P (phase pits) to detect reduction in quantity of reflected light which occurs due to diffraction, scattering or variation of optical constant of the pit, whereby information corresponding to presence or absence of the pits is obtained. More specifically, when the spot of the read-out laser light is irradiated onto the pit, as shown in FIG. 1A, the quantity of return light which is reflected is small due to scattering or the like. On the other hand, when the spot of the read-out laser light is irradiated in a gap between pits, i.e., off the pit as shown in FIG. 1B, the quantity of the return light is large. Information recorded on the optical disc is read out utilizing the difference in quantity of the return light as described above. However, in such cases, a reproduction resolution of the optical disc is restricted by the wavelength .lambda. of the read-out laser light and the numerical aperture NA, and therefore information having spatial frequency higher than the spatial frequency fc=2NA/.lambda. cannot be reproduced.